The Welcome message is displayed on the home page every time someone logs in. To change this default message go to Settings > Templates and choose the welcome_message template. You can use the template tag {USER_NAME} to display the current users name.

There is a more advanced method of setting the Welcome message to be different depending on the current users role (e.g. Staff member, Customer, etc..). Create the User Role first (Settings > User Roles) and then create a new template (Settings > Templates) called welcome_message_role_123 where 123 is the role_id you wish to target (visible in the URL when editing the role).